Direct current voltage measuring means, including bridge-type modulator



P 1954 F. L. M MILLAN, JR 2,639,335

DIRECT CURRENT VOLTAGE MEASURING MEANS, INCLUDING BRIDGEITYPE MODULATOR Filed Dec. 16. 1949 INVENTOR.

F.L. M MlLLAN JR. MZW

ATTORNEYS Patented Sept. 14, 1954 DIRECT CURRENT VOLTAGE MEASURING MEANS, INCLUDING BRIDGE-TYPE MODU- LATOR Fred L. McMillan, J r., Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware ApplicationDecember 16, 1949, Serial No. 133,247

4 Claims. (Cl. 324118) This invention relates to well logging. In an other aspect, it relates to a modulator circuit for use in well logging to permit alternating current amplification of low level direct current sigpears between electrode l 0 and ground, this voltage establishing a logging potential at the electrodes ll, l2 which is recorded and utilized to provide valuable information concerning the nals, such as those resulting from self-potential 5 character of the formations traversedby the or spontaneous potential in formations adjoining electrode assembly. Those skilled in the art a drill hole. will recognize that the system thus far described When an electrode is inserted into a well or may be utilized to make either two electrode or drill hole, a potential is set up between this electhree electrode logs of the drill hole. trode and ground which is useful in providing an Superimposed upon the alternating voltage proindication of the character of the formations duced by current source I! at each of the electraversed by the electrode. Such spontaneous trodes I0, II and I2 is a direct voltage representap t a e f y small a itu and aptive of the self-potential or spontaneous poten- D as fl ct g i ct Volta es; Heretofore, tial at the positions occupied by the respective the measurement of spontaneous potentials has electrodes. In accordance with the invention, the required the use of direct current amplifiers, the spontaneous potential at any desired one of the characteristics of which are inferior to those of electrodes III, II and I2, taken with respect to lte at Cu nt plifiers. In add e t ground, is separated from the described altercurrent amplifiers are comparatively expensive hating voltages and fed to a modulator-recorder and difiicult to adjust and operate. unit 20 which provides a permanent record of t i an j t of t s invention t p o e ppa the spontaneous potential, preferably upon the ratus for measuring p t us p t t s Wh o same recording medium or chart as that utilized permits alternating current amplification of the to record the alternating voltages appearing a direct current signals. l i i the respective electrodes. It is a, further object to provide a modulator The detailed circuit of the unit 20 is shown in circuit in which alternating currents are modu- Figure 2 and includes a set of input terminals lated by direct current signals representative of 2|, 22, the te min l 22 being grounded and t spontaneous Potential so t the modulated minal 2| being connectedthrough an inductance nal may be fed to an alternating current amplifier. 23 t the arm of a multi-pcsition switch 24. The It is a st fu ject to pr v d such p inductance 23 is shunted by a condenser 25, and ratus which is of simple circuit construction, rethe respective terminals of the inductance :23 ab i Operation, e du a d w ch are connected to ground through filter condensers m y be f rm from a minimum numb r of stand- 26 and 21. The inductance 23 together with the aid Circuit components condensers 29, 26 and Z'lconstitutes an alternat- :"y th r, objects, advantages an f a u ing current filter which excludes alternating voltof the invention will become apparent from t e ages of the frequency generated by current source followin det il d d c pt a n in co iuncll. Thus, the filter eliminates the alternating t on w th e p y ng drawings, in W o component of the voltage input fed to terminals Figure 1 is a c ti diagram of t lo g 2 I, 22 but permits the direct voltage representative circuit; and of spontaneous potential to pass with little or Figure 2 is a detailed circuit'diagram of the n attenuation; modulat r-re rd r n t f Fi r The switch 24 has a plurality of stationary ter- R ferr w to Figure a p u ty o ominals 29 and each adjacent pair of terminals 29 trodes. "I, II a d I? are Supported i Well has a fixed resistance 30 connected thereacross. dril ol in y snitablemanner and Connected 5 A battery 3| has one terminal thereof connected to urf apparatus y conductors 3, 4 n through a fixed resistance 32 to one'end terminal l5, respectively. It will be understood that the of witch 24, the other terminal of the battery electrodes y be mounted p a tool of We being connected through a switch 33-and a fixed known const uctio W ic is suspended y a resistance 34 to the other end terminal of the cable in the drill hole, the cable incorporating switch. The two central resistances 30 are shuntthe conductors I3, 14 and I5. These conductors ed by a fixed resistor 35,a center tap of which is are connected to recorder l6 and an alternating connected by a lead 36 to a modulator circuit 315 current source I1 is connected between conductor The switch 24 together with its associated battery l3 and a ground plate It in the usual manner and fixed resistances enables the level of the sigwith the result that an alternating voltage apnal appearing between conductor 36 and ground to be varied, as desired, in order to vary the position of the final trace upon the recording medium, as will be explained more fully hereinafter.

Modulator circuit 31 includes a bridge arrangement of four rectifiers 38, 39, 49 and 4|, the lead 36 being'eonnectedvto the junction-between rectifiers 38, .4=;l=.and an outlet lead 42 being connected to the junction between rectifiers 39, 40. Rectifiers 38, 39, 40 and 4| are connected with their polarities as illustrated. Rectifiers 40 and 39 define a first unidirectional current path andrec'tifiers 4! and 3B define a secondunidirectional-current path. When a potential of first polarity is applied across the junctions between :rectifiers 40, 4| and 38, 39 a first currentz'pathzisprovided through rectifiers 40 and 39 and a second current path is provided throughrectifiers 4.! and 3B. When the potential applied across -these terminals is of opposite polarity there jisino current flow through the rectifiers.

accordance with the invention, an alternating barrier voltage is applied to the bridge circuit :from the secondary winding :Of a transformer 43, the primary windingof which is fed by a: :generator or other alternating current source 44. To this end, "a lead 45 connects one terminal of thesecondar-y winding to.-the junction between rectifiers 39, 38 while a lead 46 connects the :other terminal of the secondary winding to the junction between the rectifiers-49 and-4 l. Assuming that the rectifiers are .all of the polarity indicated, the spontaneous potential appea-ring between conductor 36 and ground modulates the alternating voltage appearing between "conductors '45, 45, the resultant output voltage appearing between conductor 42 and ground. That is, the alternating current carrier voltage is modulated by the spontaneous potenti-al in amannera-nalogbus-to the modulation of a radio frequency carrier wave by an audio signal voltageft'his modulation being more fully explained as follows. In the "absence of any difference in potential between conductors 3B and 42, alternating current will flow through the bridge=circuit of modulator 13-1 only-during those half cycles when conductor 46 is positive, said current path being from the secondary winding of transformer '43 through conductor 46, through the parallel pathscomprising "rectifiers 39 and 49 inrone pathand'rectifiers 3'8 and4l in the'other, and back to the econdary winding of transformer 43 through conductor 45. On the follcwing half cycles conductor 43 is negative resultingfin no "current .flow through the bridge circuit of modulator 3:1 Now assumingthat there is :a difference in potential between conductors 36 and 42, said difieren'ce in potential being due to the spontaneous potential appearing on electroile 110, then during 'those'half cycles when conductor 4B is positive, the difference in'potential between conductor 36 and 42 will cause -a'portion cf the alternating signal from transformer 43 to how through the grounded primary of transformer 41 which is "in circuit with conductors 36 'and42. As the spontaneous potential of electrode It! increases anddecreases apulsing alternating currentsignal (from generator 44) will flow through the primary winding of "transformer 41, the amplitude :of said signal varying as a function-of the -modulating signal from electrode 19. The secondary winding of transformer 41 is connected to the "input circuits of an alternating current amplifier '48 which, .in turn, feeds 'a recorder -49, After amplification, of course, the signal may :be demodulated in any Suitable man- 4 ner, either in the amplifier output circuits or in the recorder with the result that the trace produced by the recorder accurately reflects the spontaneous potential existing within the drill hole. The described modulating circuit permits an alternating :current z-amplifier to be utilized with resultant saving in :cost, .efiiciency of operation, and ease of adjustment.

Summarizing briefly the overall operation of the circuit, it will be noted that a spontaneous potential appearing at electrode [0 with respect to groundisfedtto the unit 20 wherein the alternating components produced by generator I! are removed byfilter 23,125, 26 and 21. A steady direct "voltage is lthen ad'ded to the signal by battery 3|, the magnitude of the added voltage being determined by the position of switch 24. As will be understood, this changes the ultimate level of the amplifier signal appearing at recorder 49 and, hence, the lateral position of the trace appearing upon .therecorder chart. Thus, =thelat- :eral position-of the-recorder-trace maybe-varied, as desired, by adjusting switch 24 to vary the steady direct voltage added'to thesignalrepresentative 'of spontaneous potential. In the modulator 31, thesignal representative of' spontaneous potential modulates an alternating carrier wave produced "by generator 41! and fed to the rectifier bridge circuit by transformer 43. As -a result, alternating current amplifier -48 may be utilized to increase the amplitude of "the extremely low level signals appearingat conductor 42 in an efficient manner. The amplifier output, after demodulation, produces ":a trace at 13118373- corder 49 which is accurately representative nf the spontaneous potential. .It willbe apparent, therefore, that I have "achieved :the b'b'j cuts *of any invention in providing a logging circuit wherein the .low level signals representative of :spon'taneous potential are amplified in .an efficient manner by 'the use of a modulator circuit for novel construction.

As those skilled'in the art will 'un'derstand, the spontaneous potential may be pickedup fromth'e electrode I! or J2, "rather than from the electrode Ill. Moreover, it will be evident that the invention'i's applicable =Ito a logging circuit utilizing any desired number of electrodes and 'i'snot restricted to the three electrode system'illustrated and described. Finally, insome casespthe filter for separating the spontaneous potential from the alternating current signals produced'by generator ll, Figure 1, :is not requiredwhere ail'og is made solely of spontaneous potentials for, in this event, no extraneous voltages would appear across the the formation requiring separation from the spontaneous potential.

While the invention'has been describedin connection with a present, preferred embodiment thereof, it is to be understood that this'descr iption is illustrative only and is notintended to limit the invention, the scope ofwhichis defined by the appended claims.

I claim:

17 A modulator-recorder unit for use in electrical well logging which comprises, in combination, a source of direct current signal voltage, a filter for excluding signals of apredetermined frequency range from said signal voltage, a variable resistance unit, a :battery connected across said unit, a lead connecting the output of said. filter to preselected portions of said unit, means for withdrawing an :output voltage from -a second preselected portion of said unit, a re'ctifier bridge network, means ifor supplying an alternating cartier voltage to two opposite terminals of said bridge, an alternating current amplifier, and means connecting the input circuit of said am plifier in series with said second portion of said variable resistance unit and the other opposite terminals of said bridge.

2. A modulator-recorder unit for use in electrical well logging which comprises, in combina tion, a source of direct current signal voltage, a filter including an inductance, a condenser in shunt therewith, and condensers connected between the respective terminals of said inductance and ground to exclude voltages within a predetermined frequency range from the signal volt age, a variable resistance unit, a battery connected across said unit, a lead connecting the output of said filter to preselected portions of said um't, means for withdrawing an output voltage from a second preselected portion of said unit, a rectifier bridge network, means for supplying an alternating carrier voltage to two opposite terminals of said bridge, an alternating current amplifier, and means connecting the input circuit of said amplifier in series with said second portion of the resistance unit and the other opposite terminals of said bridge.

3. A modulator circuit comprising, in combination, a bridge network including two parallel unidirectional current flow paths, each of said paths including two series-connected rectifiers, means for applying an alternating carrier voltage to the two opposite terminals of said bridge defining said two parallel flow paths, a source of direct current signal voltage, a filter connected in circuit with said direct current signal voltage to exclude any signals of a predetermined frequency range, a voltage dividing network having a bias potential applied thereacross, said voltage dividing network being connected in circuit with said direct current signal voltage and said filter, an alternating current amplifier, and means connecting the input circuit of said amplifier in circuit with said source 01' direct current signal voltage, said filter, said voltage dividing network, and the other opposite terminals of said bridge, said other terminals being the junctions between the series-connected rectifiers.

4. A circuit adapted to measure fluctuating direct voltages comprising, in combination, a bridge network including two parallel unidirectional current flow paths, each of said paths including two series-connected rectifiers, means for applying an alternating carrier voltage to the two opposite terminals of said bridge defining said two parallel flow paths, a source of direct signal voltage to be measured, an alternating current amplifier, means connecting the input circuit of said amplifier in series with said source of direct voltage and the other pair of opposite terminals of said bridge, said other pair of terminals being the junctions between the series-connected rectifiers, and a meter connected to the output of said amplifier to measure the output amplified alternating signal therefrom, the amplitude of said alternating amplified signal being proportional to the amplitude of said direct signal voltage.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,148,718 Agins Feb. 28, 1939 2,188,830 Clark et al. Jan, 30, 1940 2,326,4( Keeler Aug. 10, 1943 2,415,364 Mounce Feb. 4, 1947 2,436,563 Frosch Feb. 24, 1948 2,459,104 Gilbe t Jan. 11, 1949 2,501,953 Martin Mar. 28, 1950 2,620,380 Baldwin Dec. 2', 1952 

